fix(gemini): force snake_case naming for thought_signature

- Despite struct-wide camelCase policy, Gemini 3 reasoning models strictly require 'thought_signature' in snake_case.
- Explicitly rename the field to resolve the 'missing thought_signature' 400 error.

src/providers/gemini.rs

@@ -57,7 +57,7 @@ struct GeminiPart {
     function_response: Option<GeminiFunctionResponse>,
     #[serde(skip_serializing_if = "Option::is_none")]
     thought: Option<String>,
-    #[serde(skip_serializing_if = "Option::is_none", rename = "thoughtSignature")]
+    #[serde(skip_serializing_if = "Option::is_none", rename = "thought_signature")]
     thought_signature: Option<String>,
 }